One of the methods of precisely measuring the position or shape of a sample (target) uses an electrostatic capacitance sensor. In this method, the magnitude of an electrostatic capacitance generated between a sensor probe and a target is detected to measure the distance between the sensor probe and the target. The electrostatic capacitance is detected as an AC impedance.
More specifically, in many cases, weak AC currents from a sensor amplifier are supplied from a sensor probe to a target, and a voltage drop by the impedance is measured. Currents flowing from the sensor probe to the target generally flow back to the other terminal of the sensor amplifier via a conductor, which is set to be almost the same potential as that of the housing ground of the system. In general, an electrostatic capacitance to be measured is a small value, on the pF order, and is readily influenced by the stray capacitance. The potential is usually set so as to reduce the influence of the stray capacitance on mounting from a sensor amplifier to a sensor probe and mounting from a target to a ground line.
The electrostatic capacitance sensor is ideally used by coupling a target sufficiently low in impedance to ground at a low impedance. For this reason, it often uses an arrangement in which a table to mount the target is made of a conductor and is connected to ground.
FIG. 27 is a view of the arrangement of a conventional measuring apparatus using an electrostatic sensor. A chuck 106 of a target 105 is made of a metal to reduce the impedance between the target and ground. In FIG. 27, reference numeral 101 denotes a sensor probe; 111, a sensor amplifier; 104, a sensor cable; 107, an insulating surface plate; and 114, a controller. The surface plate 107 and chuck 106 constitute a table.
In some tables, however, there may be cases wherein it is difficult to join a target to ground at low impedance. These cases include, for example, a case wherein the shape of the target, the shape of the table, or the material for the table, is limited. In this case, conventional methods cannot couple the target to ground at a low impedance. As a result, the stability or precision of measurement may degrade or decrease, or the target may cause an offset.
A further problem occurs in measuring the surface position of the target. To measure the surface position of the target while the table is connected to ground, the internal impedance of the target needs to be sufficiently low or the individual difference needs to be small. This is because high internal impedance or a large individual difference causes a voltage drop inside the target, producing an error in a measurement value.